Generally, a combination of a hydraulic motor and a reduction gear is conventionally used as a swing actuator that is a swing mechanism of construction machinery. Moreover, the combination of the hydraulic motor and the reduction gear is also used as a swing mechanism of a crane or a winch of a ship.
When such a swing mechanism using the hydraulic motor slows down, it needs to release rotational energy accumulated in inertial bodies (such as a boom, an arm, and a bucket) during a swing operation. Therefore, a relief valve is provided in a hydraulic circuit, and the accumulated energy is converted into heat and the heat is released by pressure generated by the relief valve when the swing mechanism slows down. To be specific, in the case of the swing mechanism, since the energy accumulated in the inertial bodies during the swing operation is abandoned as the heat when the swing mechanism slows down, the energy use efficiency is low.
Similarly, in the case of the winch, since the relief valve provided in the hydraulic circuit converts the potential energy of a cargo into the heat and releases the heat when the winch rolls down, the energy use efficiency is low.
Here, in this kind of technology, proposed is an invention for improving the energy use efficiency. One example is an invention in which a surplus pressure fluid of a pump driven by a motor generator connected to an engine is accumulated in an accumulator, and the pressure fluid is effectively utilized according to need (see Document 1, for example).
Another example is an invention in which the hydraulic motor and the electric motor are used at the same time to drive a swing drive unit of a swing machinery, and when the swing drive unit steadily swings and slows down, the electric motor is caused to carry out electric power regeneration to accumulate regenerative electric power in a capacitor (see Document 2, for example).
Further, proposed is an electro-hydraulic hybrid motor configured such that a hydraulic pump is driven by the electric motor, and the hydraulic motor is driven by delivery fluid from the hydraulic pump (see Documents 3 and 4, for example).    Document 1: Japanese Laid-Open Patent Application Publication No. 2007-10006    Document 2: Japanese Laid-Open Patent Application Publication No. 2005-290882    Document 3: Japanese Laid-Open Patent Application Publication No. 8-251867    Document 4: Japanese Laid-Open Patent Application Publication No. 8-251868